The production of oil and gas from wells drilled into the ground frequently requires the use of a mechanism to elevate fluid from the bottom of the borehole to the surface. A commonly employed mechanism comprises a reciprocating downhole pump driven by a motor, often a pump jack, at the surface. Such pumps typically have a stationary standing valve positioned at the bottom of a string of production tubing near the producing perforations of the well. A traveling plunger assembly in a hollow cylindrical barrel positioned above the standing valve has a traveling valve assembly that opens on the down stroke of the plunger and closes on the upstroke. In contrast, on the upstroke of the plunger the standing valve opens allowing fluid to fill the space below the plunger in the cylindrical barrel, and on the down stroke the standing valve closes trapping the fluid drawn into the cylindrical barrel during the upstroke of the plunger.
The plunger assembly is attached at its top end to a sucker rod which is actuated by the pump jack at the surface. In this manner, each upstroke lifts a column of fluid towards the surface while each down stroke charges the space immediately above the plunger with a new column of fluid ready for the next upstroke. There are numerous variations and configurations of this type of pump but in each instance, the consistent opening and closing of the traveling valve with the down and up strokes of the plunger is essential to the efficient pumping of oil up the production tubing.
The traveling valve assembly in such reciprocating pumps commonly consists of a ball and seat type valve comprising a ball resting on seat within a valve cage. On the down stroke, the movement of the valve assembly through the fluid and the incompressible nature of the liquid trapped between the traveling valve and the standing valve lifts the ball from its seat thereby opening the valve. On the up stroke, the hydrostatic pressure of the fluid and the movement of the valve assembly through the fluid forces the ball down onto the seat closing the valve. Other types of valves employing similar actuating mechanisms on the up and down strokes are employed, including flapper valves.
Although the reciprocating pump described above is reliable and commonly used, there are production circumstances that can render its use problematic and inefficient. In particular, wells that produce dissolved gases, such as natural gas, along with the oil and water can cause problems. Upon production, the dissolved gas can break out of the solution. Gas that is produced is easily drawn through the standing valve on the upstroke of the plunger. However, on the down stroke when the standing valve is closed and the liquid body below the traveling valve is normally expected to force the traveling valve open, gas between the traveling valve and the standing valve will compress and the greater force of the hydrostatic head of the fluid above the traveling valve will keep it closed. On the following upstroke, the compressed gas between the traveling valve and the standing valve expands to fill the enlarged space and this prevents the flow of more fluid through the standing valve into the cylindrical barrel. In this manner, the upstrokes and down strokes of the pump simply result in the repeated compression and expansion of trapped gas between the standing valve and the traveling valve and the pumping of fluid is prevented. This phenomenon is referred to as “gas locking”.
An associated problem is “fluid pounding” which occurs when the space in the cylindrical barrel below the traveling valve is partially filled with fluid and partially with gas. The consequence of such a composition in the barrel cylinder is that the plunger forcefully enters the fluid level part way through the down-stroke. This causes undesired vibrations, or ‘pounding’, through the production string leading to mechanical failure and expedited wear.
There are prior art solutions to the problem of gas locking which usually involve some form of gas equalizer comprising a probe or piston that mechanically actuates the valve of the traveling valve assembly. This mechanical opening of the valve overcomes the hydrostatic pressure above the valve and allows any produced gas to flow through the traveling valve assembly thereby eliminating a gas seal from forming below the traveling valve. U.S. Pat. No. 4,867,242 to Hart and U.S. Pat. No. 5,382,142 to Spears are examples of prior art solutions to the problem of gas locking. Both have an actuated piston that engages and unseats the ball in the traveling valve assembly. However, in these prior art solutions, the produced fluid passes through a passage in the center of the apparatus. Whenever there is a narrowed passage or channel for the fluid to pass though that is smaller in diameter than the valve opening, or the internal diameter of the ball and seat in the case of a ball and seat type valve, there is a resulting pressure drop in the fluid which promotes the break out of scale and gas from the fluid. Scale build up over time causes the ports to become more restricted causing further pressure drop, loss of production and pump failure. Gas break out due to poor flow design can result in unwanted production problems such as fluid pounding.
Furthermore, the prior art gas breaking solutions are relatively complex and expensive to manufacture and implement. The replacement and maintenance of the prior art gas equalizers are also relatively time consuming and expensive. Furthermore, they are difficult to adapt for use with the many varieties and models of downhole pumps being employed in the field.
Therefore, what is required is an improved apparatus for use with a traveling valve assembly of a downhole pump for releasing gas to prevent gas locks. It would also be preferable if the apparatus mitigated the limitations of the prior art and had an improved flow design to mitigate the problem of pressure drop as the produced fluid moves through the traveling valve assembly.